The flight-proven General Purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-RTG) has a unit power output of about 290 Wdc using heat from a radioactive decay of 238Pu included in eighteen General Purpose Heat Source (GPHS) modules at the beginning of life (BOL). The GPHS-RTG typically weighs about 56 kg and has a specific power of 5.4 W/kg. With a nominal thermal power for a single GPHS module of 250 Wth at BOL, the GPHS-RTG has a system efficiency of 6.7%. Conventionally, the GPHS-RTG powered large space exploration missions launched in the decade of the 1990's (e.g. Ulysses, Galileo, and Cassini) as well as the New Frontier class Pluto-New Horizons mission in 2006.
Since then, space programs (e.g. National Aeronautics and Space Administration (NASA)) have focused efforts on developing smaller Radioisotope Power Systems (RPS) with multi-mission capability, capable of operation in space and in planetary atmosphere environments.
For example, an Advanced Stirling Radioisotope Generator (ASRG) provides high fuel efficiency relative to a comparable Radioisotope Thermoelectric Generator (RTG). Furthermore, due to a limited inventory and future production rate of 238Pu, there is increased incentive to use the ASRG unit. However, the nominal power output of the ASRG limits its application to lower power uses, (e.g. 140 Wdc).